Refactor to use gdk_threads_add_idle instead of gdk_threads_enter/leave

[trupples]

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[dNechita]

This approach seems fine. There are some calls to gdk_threads_enter()/leave() in this file but it's probably due to this PR still being WIP.
My advice is to test this functionality on Windows as well.

[nunojsa]

LGTM... As @dNechita pointed out, we need to make sure it works on all supported OSes and to remove all the calls to gdk_threads_enter()/leave() also in the plugins (in a patch per plugin)

[nunojsa]

nit: sizeof(*params)